In vacuum molding of objects of synthetic resin, a dismountable mold is used with reinforcing materials placed in the mold. The vacuum molding is effected in such a way that a vacuum is applied to the cavity through one or several connections arranged at the upper portion of the mold. Ready-mixed synthetic resin in the form of monomer and hardening agent are fed into the lower portion of the mold. The synthetic resin rises in the mold by way of the applied vacuum.
The first synthetic resin supplied rises until it reaches the uppermost part of the mold. When large objects are manufactured, the rising of the resin takes a long time. During this time the synthetic resin will become viscous, producing lumps and plugs of synthetic resin in the cavity, preventing the continued rising of the synthetic resin in the cavity. Thus, it is difficult to manufacture large, particularly shell-like objects, with little thickness relative to the surface area, such as domes, boat hulls, or car bodies, with vacuum molding.
Known methods for vacuum molding avoid the above-mentioned problems by using only small amounts of hardening agent in the monomer and subsequent heating of the mold after the mold has been filled with synthetic resin for hardening. However, this method is circumstantial and difficult to carry out when large objects of synthetic resin shall be vacuum molded.
One of the great advantages of vacuum molding is that it is not harmful to the environment, since no emission of monomer to the atmosphere takes place. An other advantage is that the molding is effected in one single working step, whereby the manufactured object becomes totally homogeneous and free from voids due to the vacuum applied.